The invention relates to the annealing of tubing and similar elongated products such as rods or wires. More particularly, it relates to the annealing of elongated products on which it is sometimes desirable to have at least selected portions of their length in an annealed state. One such product is straight lengths of finned tubing to be used in heat exchangers. Such tubing has been conventionally processed in short lengths cut from a straightened portion of a large coil which has been annealed. The short lengths have a long mandrel inserted in them and are then positioned in a fin rolling apparatus which is capable of selectively moving the finning members into and out of contact with the tubing so that unfinned lands and ends can be provided. When the unfinned portions are in an annealed state, the tubes can be more easily assembled into the tube sheet header and baffle portions of heat exchangers by internal expansion techniques.
Techniques previously used to anneal such tubes have included both batch and continuous techniques. In the batch technique, the entire coil or individual tube length must be placed in a furnace for an extended period. The process is very time-consuming, requires very expensive capital equipment, and also utilizes a large amount of energy. Continuous annealing techniques have also been developed, one example of which can be seen in Herren et al U.S. Pat. No. 3,518,405. In this apparatus, the tubing is bent partially around each of a spaced pair of current-carrying electrode wheels which cause the portion of the tubing between the wheels to be heated. The electrode wheels are typically made of graphite which wears relatively rapidly, thus causing a substantial expense in maintenance, electrode replacement and downtime. Another example can be seen in Judd, U.S. Pat. No. 4,309,887 wherein the entire tube is continuously annealed by an induction heater.